Federally numbered route didn't follow the existing main corridor

[NE2]

There are two cases I can think of where a state got AASHO or the feds to align a route on a new corridor rather than the existing route used by most traffic:

US 20 in New York: if my history is correct, NY had already built the Mohawk River (NY 5) corridor to what were at the time high standards by the early 1920s. They requested their east-west 7% federal aid highway on a more southerly corridor, in order to receive federal funding for a new road. The U.S. Highway system was laid out primarily on federal aid highways, so this new bypass became US 20. NY 5 never received a U.S. Highway number because New York was not Pennsylvania.

I-5 in California: similarly, CA was already building a freeway along US 99, and wanted federal funds for a westerly bypass through the Central Valley.

Can anyone think of others, where if the state had not been forward thinking, the route would have been instead designated on an existing facility?

[skluth]

US 10 isn't half as busy an east-west corridor as WI 29 between Lake Michigan and the Twin Cities. They're roughly parallel and only about 20 miles apart.

An argument could also be made for WI 57 between Green Bay and Milwaukee. Before I-43, it was easier to drive via WI 57 between the two cities than use either US 41 or US 141. US 41 traffic was mostly the Fox Cities residents and businesses while US 141 served Manitowoc and Sheboygan. A bigger version of the WI 26 shortcut through Rosendale to Madison.

[TheStranger]

To some extent this happened with US 101 and the 1934 Route 150 (now parts of Route 246, Route 154, and current Route 150) in Santa Barbara County; Cahighways notes that the original 150 corridor was the primary route in the region in the late 1890s up until US 101 was built.

[sparker]

The alignment of I-40 has a few such deviations from the original existing route; the westernmost is right in CA, where instead of following US 66 through Amboy (it followed the then-ATSF tracks) it cut across on a new-terrain route north of the original, cutting about 30 miles off the total between the endpoints of the divergence.  This was repeated in AZ between Kingman and near Seligman; again original US 66 followed the RR main line, while I-40 cut across new terrain to the south.  AZDOT was fortunate that the west end of that new alignment was right along AZ (now US )93, so construction was simplified to some degree.  The last was the TN/NC routing of I-40 between Knoxville and Asheville through the Pisgah Gorge, well to the west of the main route between those two cities, US 70 and US 25W/US 25 along the French Broad River valley (also the locale of the Southern Railroad line through that mountain range).  The first two were straightforward mileage-reducers; the latter may have actually increased the actual mileage between the two main cities along I-40 in comparison to the existing US 70 alignment.

[Hot Rod Hootenanny]

US 42 vs OH 3 between Cleveland, Columbus, & Cincy (US 42 skipped C-bus)
US 21 vs OH 8 between Cleveland and south (US 21 skipped Akron & Canton)

[Max Rockatansky]

I-10 was built over open desert west of Phoenix rather using US 60/70.

[TheStranger]

I-10 was built over open desert west of Phoenix rather using US 60/70.

In that vein, I-8 following AZ 84 instead of US 80 east from Gila Bend to Casa Grande (and I-10 following AZ 84 east to Tucson) could be seen similarly:

I-8/I-10 using the more direct state route corridor instead of the existing US route.  (I-10 also uses much of the former AZ 93 route from Phoenix to Tucson, rather than today's AZ 79 which is the former US 80/89 between the two cities)
US 80 taking the indirect route to serve Phoenix rather than the direct east-west trajectory towards Casa Grande and Tucson

[BrianP]

The main corridor between Baltimore and Annapolis was MD 2. MD 10 is a freeway that was planned to run between Baltimore and Annapolis to relieve MD 2.  But instead of finishing that freeway, a new routing was made for the southern part of what became I-97. The northern part used the existing MD 3 Glen Burnie bypass.  And the central part upgraded some more of MD 3.

And I-95 was built as separate highway instead of using the B-W parkway between Baltimore and Washington.

[Max Rockatansky]

I-10 was built over open desert west of Phoenix rather using US 60/70.

In that vein, I-8 following AZ 84 instead of US 80 east from Gila Bend to Casa Grande (and I-10 following AZ 84 east to Tucson) could be seen similarly:

I-8/I-10 using the more direct state route corridor instead of the existing US route.  (I-10 also uses much of the former AZ 93 route from Phoenix to Tucson, rather than today's AZ 79 which is the former US 80/89 between the two cities)
US 80 taking the indirect route to serve Phoenix rather than the direct east-west trajectory towards Casa Grande and Tucson

I had all those listed up but more or less stuck to the initial premise about entirely new corridors.  To that end Arizona went through a hell of a large number of improvements mid-20th Century that made US 80 largely obsolete as a through route across the state. 

[TheHighwayMan3561]

MN: I-90 between Dexter and La Crosse deviated away from US 16 where no state highway previously existed, to bypass the difficult terrain now-MN 16 traverses between Lanesboro and La Crosse. East of Rochester 90 very loosely shadows US 14 to La Crosse where 90 rejoins the former US 16 alignment.

[US 89]

I-15 deviated well away from US 91 between St George, UT and Littlefield, AZ, in favor of building a highway directly through the Virgin River Gorge.

One of the better-known cases is probably I-70 in Utah west of Green River. Initial plans had it following the US 6 (then also US 50) corridor northwest to Salt Lake City, but it was built heading west and southwest to Cove Fort instead. Parts of this extension followed the corridors of the existing SR-13, US-89, and SR-10, but the portion between Fremont Junction and Green River was an entirely new corridor. US 50 was rerouted onto it in 1976 when it was a two-lane interstate, but it wasn't fully four-laned until 1990.

[sparker]

I-90 forged a more straight-line course in WY from its alignment along US 87 east to Gillette; whereas US 14 & 16 take an arching path north of the I-90 alignment (like many others out West, generally following a rail line) from, respectively, Sheridan and Buffalo, I-90 just struck out east across the hills in a more or less direct line from Buffalo (and the I-25 interchange) to Gillette. 

[NWI_Irish96]

It may have been the more traveled route at the time it was originally designated, but US 20 between South Bend and Rolling Prairie follows a 2-lane and less direct route compared to the current IN 2, which is 4 lanes and more direct.

[hbelkins]

I-64 doesn't follow US 60 between Charleston and Greenbrier County, and also between Lexington and Richmond, Va.

[corco]

I believe I-80 between Rawlins and Laramie, Wyo. fits the bill - there's even a random stretch of divided 30/287 where they thought the road would eventually be I-80 before deciding on a southerly corridor.

[Max Rockatansky]

I-4 diverged significantly from US 92 between Lakeland and Orlando Via a new corridor in the Green Swamp. 

[CNGL-Leudimin]

I-70 across the San Rafael Swell. US 50 is now in that route too, but I found it to be a deceiver, as no roads crossed the swell before the freeway was built, and US 50 was previously concurrent with US 6 all the way across Utah (thus making either one a useless route in that state from my viewpoint).

[Beltway]

I-64 doesn't follow US 60 between Charleston and Greenbrier County, and also between Lexington and Richmond, Va.

I don't think that US-60 ever was the main corridor between Lexington and Richmond, Va.

At most it would have shared traffic duties along with the US-360/US-460/US-501 corridor and the US-250/US-11 corridor.

[hbelkins]

I-64 doesn't follow US 60 between Charleston and Greenbrier County, and also between Lexington and Richmond, Va.

I don't think that US-60 ever was the main corridor between Lexington and Richmond, Va.

At most it would have shared traffic duties along with the US-360/US-460/US-501 corridor and the US-250/US-11 corridor.

If there are no interstates, and I'm in Lexington and want to drive to Richmond, I'm not going to go out of my way north on US 250, or south on US 501, to then head east. Especially on 501, which is a horrid drive from Lexington to Lynchburg.

[froggie]

^ It's worth noting that US 60's original routing went south with US 11 to Natural Bridge, then via the VA 130 corridor to Glasgow, US 501 to Lynchburg, US 460 to Burkeville, then US 360 to Richmond.  It wasn't until 1933 that it was moved onto (more or less) its current corridor between Lexington and Richmond.

[DandyDan]

I-35 between US 20 near Williams, IA and Albert Lea, MN first diagonalizes and then turns north between US 65 and US 69.

I-380 between Cedar Rapids and Waterloo, IA follows a diagonal route on the east side of the Cedar River, whereas US 218 was on the west side of the river.

[NE2]

I guess what I was getting at is that the unused main corridor was already seeing some improvements of the form that would be expected with the new designation (paving on NY 5, freeway on US 99).